There are a variety of applications for optical systems that combine light from two different sources, such as from multiple displays or a display and the environment. One common application is a camera viewfinder that superimposes data from a digital display onto the image received through the camera's lens. Other applications are head, e.g., helmet, mounted display systems. These systems typically include image displays such as micro-flat-panel display devices based on organic light emitting diode (OLED) technology or active-matrix liquid crystal display (AMLCD) technology, for example. The systems also include eyepieces that couple the light from the display devices and light from the environment to the users' eyes. The head/helmet-mounted display systems are used in the military, in field operations and simulations, medical procedures, and industrial maintenance operations. There are now also opportunities to use head-mounted displays for game consoles, cellular phones, portable video players, and other entertainment and communication devices.
“Solid-Schmidt” prism eyepieces are a particularly attractive optical design approach for these optical systems that combine images from two different sources. The “Solid-Schmidt” design is well known in the prior art, using a powered mirror surface in conjunction with a double-pass beam splitter surface. See, e.g., U.S. Pat. No. 5,696,521 to Robinson, et. al. This optical system provides excellent image quality by means of having all the primary optical power provided by a single optically powered mirror surface. However, because of the reflective nature of the mirror element, it is necessary to utilize some form of beam-splitting components such that the light may pass in the desired direction.